Add info about `solid` option for `indent_guide_options`

[jrappen]

Make clear that solid can be added:

    // hidden
    "indent_guide_options": [],

    // normal
    "indent_guide_options": ["draw_normal"],

    // active
    "indent_guide_options": ["draw_active"],

    // solid hidden (doesn't make sense to set it this way)
    "indent_guide_options": ["solid"],

    // solid normal
    "indent_guide_options": ["draw_normal", "solid"],

    // solid active
    "indent_guide_options": ["draw_active", "solid"],

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compare sublimehq/sublime_text#2126

[FichteFoll]

I think this should be fixed upstream, since the original setting is much more likely to be seen by users than our popup. We would only be covering up this oversight in one specific instance (and our completions).